The present invention is described herein in the context of a dimming system for a lighting load, but is not limited thereto. The present invention has applicability in any AC phase control system where it is desired to minimize undesired variations in the power delivered to a phase controlled load caused by a noisy or unstable AC source, especially at low levels of delivered power.
Most lighting control systems that have a dimming capability employ a thyristor in series with the AC lighting load to effect the dimming function. Dimming is performed by altering the conduction angle of the thyristor, usually by delivering a trigger signal to a gate of the thyristor such that the timing of the trigger signal varies with the selected dimming level. In a typical forward phase control system, generation of the trigger signal is synchronized with the AC line voltage (the fundamental frequency waveform of which is sometimes referred to herein as "the AC fundamental") such that, some time after a zero crossing of the AC line voltage is detected, the trigger signal is generated, the gate of the thyristor is energized, and the thyristor conducts for the remainder of the AC half cycle. During the time interval between the detection of the zero crossing and the generation of the trigger signal, the thyristor is non-conducting (during which time no power is delivered from the AC source to the load), and usually this time interval is altered in response to adjustment of a dimming knob or slider by a user, or in response to changes in a dimming signal level. Altering this time interval thus alters the conduction angle of the thyristor, and hence alters the RMS power delivered to the load. See commonly assigned U.S. Pat. No. 5,430,356 entitled "Programmable Lighting Control System With Normalized Dimming For Different Light Sources", the entirety of which is incorporated herein by reference.
At low levels of delivered power (i.e., conduction beginning at phase angles greater than about 135.degree. for each first half cycle, and greater than about 315.degree. for each second half cycle, of the AC fundamental), even a small variation in the conduction angle usually represents a relatively large variation in the percentage of the total delivered RMS power. At these low power levels, any variation of the conduction angle, whether between AC cycles or over periods of time, can be manifested as annoying and unacceptable intensity changes, including visible flickering of the light source. Since the conduction angle is dependent on the detection of the zero crossing, it is crucial that zero cross detection be accurate and reliable. AC line conditions are rarely ideal, and less than ideal conditions can cause inaccuracy in the detection of zero crossings, with consequent intensity variations and/or flickering, as well as other problems, especially at low levels of delivered power.
The prior art has recognized that one condition that can cause intensity variations and/or flickering is intermittent and/or periodic electrical noise on the AC line. For example, voltage "spikes" can be imposed on the AC line by the switching on and off of heavy equipment such as large motor loads. See FIG. 1. Electrical noise on the AC line can be incorrectly interpreted by the dimming circuitry as zero crossings of the AC fundamental, and these false interpretations can lead to premature and/or erratic conduction of the thyristor. The prior art has also recognized that another condition that can cause intensity variations and/or flickering is distortion of the AC waveform, which can be caused by the mere presence of other loads on the line. Distortion may be characterized by a "bumpy" or "wavy" AC waveform, i.e., one that is not a smooth sinusoid. See FIG. 2. This "bumpiness" can also move relative to the AC fundamental, i.e., it is not synchronized to it. Distortion can also cause false zero crossing detection. One common prior art solution to the problem of detecting actual zero crossings in a noisy and/or distorted AC line is to employ a phase locked loop (PLL) to generate a signal internal to the dimming system having a frequency that is intended to track that of the AC fundamental. In this system, the internal signal is a new signal generated by the PLL that is intended to replicate the AC fundamental. The zero crossings of the internal signal are detected, and since it is relatively free of noise and distortion, zero crossing detection is relatively straightforward.
The prior art has also recognized (separately from the problems of noise and distortion) that frequency variations can occur in the AC line. A common prior art solution to the problem of detecting zero crossings in an AC line having unstable frequency is to sample the AC line during a small "sampling window" (e.g., 500 microsec. wide) at periodic intervals. In this type of system, known as "window detection", a sample timer is set to open the sample window just before the next zero crossing of the AC line is expected, e.g., for a 60 Hz line, the sample window is opened at 8.33 msec. intervals. During the time that the sample window is open, the AC line is monitored for a zero crossing; the AC line is not monitored for zero crossings between sample windows. Any zero crossing that is detected after the sample window has been opened can be taken as the actual zero crossing of the AC line, and the sample timer is reset. In a prior art system that has made and sold by the assignee hereof as the Grafik Eye 4000 Series dimming panel, the last zero cross detection is used as the actual zero cross crossing of the AC line. The window detection method can detect zero crossings in an AC line of unstable frequency provided that the change in period is not so substantial that the actual zero crossing falls outside of the sampling window.
The prior art, therefore, has attempted to detect zero crossings by either operating upon a separately generated signal that is intended to replicate both the phase and frequency of the fundamental of the AC line, or by operating upon the AC line itself.
Another condition that can cause intensity variations and/or flickering is changes in the RMS voltage of the AC line. Changes in the RMS voltage of the AC line can be caused by the presence of harmonics of the AC fundamental on the AC line; the presence of these harmonics changes the shape of the AC line voltage waveform from a pure sinusoid to, e.g., a generally sinusoidal waveform having flattened peaks, rather than round peaks. See FIG. 3. Changes in the RMS voltage of the AC line will cause intensity changes in the lighting load because such changes result in variations of the total power delivered to the load, irrespective of when the zero crossings occur.
The prior art has failed to recognize that the conditions of noise/distortion on the AC line, on the one hand, and frequency variation of the AC line, on the other hand, may be simultaneously and/or alternatively present. The prior art has also failed to recognize that, in addition to the presence of these conditions, the condition of changing RMS voltage may also be simultaneously and/or alternatively present. To make matters worse, all of these conditions may be variably and intermittently present on the AC line, and these and other line conditions may be constantly changing. A condition that is present at one moment may be gone or replaced by another at the next moment; one combination of conditions may exist at one moment and be replaced by another combination at the next moment; and/or all or none of these conditions may exist at any given time. Thus, in this respect, the condition of the AC line can be extremely dynamic. No prior art has attempted to address the problem of detecting zero crossings in the AC line under combinations of these conditions, in part, because the prior art has not recognized this dynamic nature of the AC line.
The applicants hereof have not only recognized the dynamic nature of the AC line, but have also recognized why the prior art is incapable of addressing all of these conditions. In particular, the applicants hereof have recognized that conventional techniques for detecting zero crossings in the presence of noise/distortion are inconsistent with conventional techniques for detecting zero crossings in an AC line of unstable frequency, and that the prior art solutions to these problems conflict. For example, the conditions of noise and distortion suggest that the AC line be integrated over a number of cycles, and therefore over a period of time, as is done when using a phase locked loop. The conditions of frequency changes and changes in RMS voltage, however, suggest an instantaneous analysis of the AC line and an instantaneous response to any frequency or RMS voltage change. Clearly, invoking a time delay and instantaneous analysis and response are conflicting solutions.
The applicants hereof have also discovered that above described PLL and window detection methods are inconsistent and conflicting solutions.
The PLL method that is effective in a noisy environment is ineffective when the AC line frequency is unstable, because the PLL effectively employs an integration technique and frequency changes in the AC line can result in a temporary phase shift between the internally generated signal and the AC line. Over time, the PLL will adjust the phase of the internal signal to create substantial coincidence with the AC line, but during the adjustment time, the conduction angle of the thyristor will vary and may be manifested as slow intensity variations in the lighting load. This condition can be aggravated if the magnitude and/or rate of frequency variation of the AC line is greater than the error correction rate of the PLL, because the PLL may then be unable to track the AC line due to timing limits in the software. In this event, the phase difference between the AC line and the internal signal will become great, and the intensity of the light source may vary significantly. Any time there is a substantial difference between the AC line zero cross and the internally generated signal of the PPL for a substantial period of time, visible changes in light will occur.
The window detection method that is effective to detect zero crossing in an AC line of unstable frequency is ineffective in the presence of noise/distortion because, when the sample window is open, any zero crossing that occurs, whether due to noise, distortion or an actual zero crossing of the AC line, can be taken as the actual zero crossing of the AC line. This type of system does not adequately distinguish noise and distortion from actual zero crossings, and can possibly aggravate flickering/intensity variation problems caused by false detection of zero crossings due to noise/distortion.
Thus, at least in the United States, where the frequency of the AC line delivered by U.S. electric utility companies has tended to be very stable, there has been no substantial need and /or effort to address the issue of frequency change in the AC line in a dimming system, and the prior art has tended to focus on the conditions of noise and distortion.
Some prior art systems gate the thyristor at regular periodic intervals, based on a selected dimming level, on the assumption that there will be no change in the timing of the zero crossings of the AC fundamental, or in the RMS voltage of the AC line after the thyristor has begun conducting. They are designed to deliver what is assumed will be a fixed amount of power once conduction begins. In these systems, the problems caused by frequency changes and changes in the RMS voltage of the AC line can be exaggerated. For example, in a given cycle, both the frequency of the AC line may change (causing a change in the time between zero crossings), and, the RMS voltage of the AC line may change during conduction. Since, once fired, the thyristor will continue to conduct until the next zero crossing occurs, the RMS power delivered to the load can vary substantially relative to a preceding or succeeding cycle.
If the integration time of the PLL is made sufficiently large to avoid the effects of AC line noise, frequency variations as small as 0.2% can be visible, and in some locations, especially in some less industrialized countries, the frequency of the AC line supplied by the electric company can change substantially more than this over very short periods of time.
Software can be employed in a dimming system to analyze the AC fundamental and address one or more of the above conditions, but a software based system can cause other problematic conditions, such as aliasing on the AC line, due to interaction between the system's sample clock and the AC line. Aliasing occurs when the waveform is under-sampled.
The prior art also includes a dimming system, known as the N-Module, that has been made and sold by the assignee hereof. A simplified diagram of a portion of the N-module is shown in FIG. 4. As shown, a transformer T1 steps down 120 VAC to 24 VAC. A 2.2 uF capacitor C1 placed across the output of T1, and before the full wave bridge rectifier (FWB), combines with the inherent leakage inductance of T1 to form an LC filter that reduces or eliminates frequencies above about 1.6 kHz. The filtered signal is supplied to the FWB, and the full wave rectified output of the FWB is analyzed for zero crossings by a zero crossing detector ZC. The output of ZC is then used for dimming purposes. This prior art does not address any of the above discussed problems. This type of system will operate satisfactorily when the AC line frequency varies, but may not operate properly when the AC line is noisy because the corner frequency is too high to cut off noise components in the frequency range 60 Hz to 1.6 KHz.
The applicants hereof have determined that a solution to the dynamic problem discussed above requires a system that has both a fast response (for dealing with frequency variations in the AC line) and a narrow bandwidth (for eliminating the effects of noise and distortion outside the range of the fundamental frequency). The present invention is directed to such a solution.